Intermediates useful in the synthesis of alpha-lipoic acid



United States Patent DITERMEDIATES USEFUL IN THE SYNTHESIS OF a-LIPOIC ACID Frederick W. Holly, Cranford, Arthur F. Wagner, Princeton, and Edward Walton and Carl H. Hoffman, Scotch Plains, N.J., assignors to Merck &, Co., Inc., Rahway, N.J., a corporation of New Jersey No Drawing. Application February 19, 1957 Serial No. 641,008

7 Claims. (Cl. 260-481) This invention is concerned with the production of novel 6,8-(hydrocarbon substituted mercapto)--oxo caprylic acids having the formula and salts and esters thereof, wherein R and R are the same or different hydrocarbon substituents such as alkyl, alkenyl, alkynyl, aryl and aralkyl groups. This invention is also concerned with novel processes for producing these compounds in which a y-(hydrocarbonsubstituted mercapto) -butyryl halides are used as. the starting materials. In addition, this, invention is concerned with novel intermediates useful in producing the 6,8-(hydrocarbon substituted mercapto)- 5-oxocaprylic acids.

This application is a continuation-in-part of our coonr-cm-omomnooon S if omcmonocnooom omomonoox 000R:

SR sn YCH SR SR coon:

coon

1 n In wherein X represents a halogen, Y represents an alkali metal, alkaline earth metal or alkoxymagnesiurn and R, R, R and R represent the same or different hydrocan bon substituents such as alkyl, alkenyl, alkynyl, aryl and aralkyl groups.

The diesters oi 4,6-(hydrocarbon substitutedmercapto)-2-carboxy-3-oXocapro-ic acids are-then converted to the corresponding 4,6-(hydrocarbon substituted mer-.

. distillative removal of the solvent.

2. capto) -3-oxocaproic acids and esters thereof according to the reactions o I oruonzcudomooom H z'sa' R onzonlonocnooom 0 iv SR SR 00.012 II \omcmonoomcoorr SR SR III IV-A wherein R, R, R and R have the significance assigned.

The 4,6-(hydrocarbon substituted mercapto) -3.-oXo-. caproic. acids are subsequently reacted in the form of an ester or salt with a suitable derivative of acrylic acid such as an amide, nitrile, ester or salt to form the corresponding derivative of 6,8-(hydrocarbon substituted mercapto)- 4-carboxy-5-oxocaprylic acid, which reaction can be ilpreviously lustrated as follows:

0 CH CH CHiiiCH CO0R +QHz=GHR SR sR I v VI.

0. cmornortiiortcmomlt s12! SR 0012 VII wherein R and R have the meaning previously assigned,

R represents an alkyl, aryl or aralkyl group or a metal and R represents the cyano, carbamyl and COOR groups wherein R has the same significance assigned herein. The fourth and last reaction comprises a simultaneous hydrolysis and decarboxylation of the derivatives of the. 6,8-.(hydr ocarbon substituted mercapto) -4-carboxy-5.-ox0 caprylic acids to produce the desired 6,8-(hydrocarbon substituted mercapto) -5-oxocaprylic acids as illustrated by i the following reaction SR SR COOR SR SR VII VIII wherein R, R, R and R have the meaning given above.

The initial reaction of this process in which an tangy-(hydrocarbon substituted mercapto)-butyryl halide. (I) is; reacted with a salt of a malonic acid diester (II) to prepare a diester of 4,6-( hydrocarbon substituted mercapto.)-. 2-carboxy-3-oxocaproic acid (III) may be convenientlyachieved by contacting the reactants in the presence of an; inert solvent. For this purpose, inert solvents such as the lower alkyl ethers like ethyl ether andisopropyl ether, chloroform, saturated hydrocarbons such. as pentane and hexane, benzene andtoluene are particularly suitable pref-- erably when substantially free of water. The mixture of: reactants in a suitable solvent is ordinarily heated. to am elevated temperature to efiect the reaction and, in: this regard, the reflux temperature may be usedwith good rea sults. Under these conditions the reaction is completed in less than one hour. Following completion of this re,-. action the desired diester of 4,6-(hydrocarbon substituted mercapto)-2-carboxy-3-oxocaproic acid, may be isolated. from the reaction mixture by conventional methods as. It is preferred, how-: ever, not to isolate this product from the, reactionmix-l turc. but, instead to immediately subject the product pres.- ent in the reaction mixture to conditions which elfect decarboxylation of the 2-carboxy group according'to meth-e ods; hereinafter disclosed.

Patented Feb. 9,. 1.960

The described reaction may be conveniently effected using any suitable, an -(hydrocarbon substituted mercapto) -butyryl halide as the starting material. Thus, the nag-hydrocarbon substituents maybe the same or diflferentalkyl, alkenyl, alkynyl, aryl and aralkyl radicals. In this regard, itis generally preferred to employ those compounds as starting materials which have identical hydrocarbon substituents. Such compounds are referred to herein as bis-substituted. In addition, the u,'y-biS (lower alkylmercapto and lower alkenylmercapto) butryl halides are particularly useful in the synthesis of ot-lipoic acid. Specific examples of" hydrocarbon substituents which may be presentlon these starting materials are methyl, ethyl, propyl, butyl, t-butyl, allyl, butenyl, pentenyl, heptenyl, ethynyl, propynyl-Z, phenyl, benzyl and similar. groups. "In. addition, this reaction is preferably achieved with halides of the type described in which the halogen substituent is either chlorine or bromine. Salts ofmalonic acid diesters which may be used in this invention that might be mentioned are the alkoxymagnesium derivatives of malonic acid diesters or the alkali metal and alkaline earth metal enolates. The

sentative of other similar compounds which are produced according to this decarboxylation reaction from the appropriate reactants are methyl-4,6-bis(ethylmercapto)-3- oxocaproate, ethyl-4,6-bis ethylmercapto) -3-oxocaproate,

ethyl-4,6-bis(allylmercapto)-3-oxocaproate, methyl-4,6-

bis(2-methyl-1-ethyl-2-butene mercapto)-3-oxocaproate, propyl 4,6 bis(3 methyl 2 ethyl 2 pentene mercapto -3-oxocaproate, phenyl-4,6-bis (phenylmercapto -3- oxocaproate, ethyl-4,6-bis(benzylmercapto)-3-oxocapro- I ate, benzyl 4,6 bis(propargylmercapto) 3 oxocapester groups of such salts may be either the same or difierent alkyl, aryl and aralkyl groups and are preferably lower alkyl groups. Some specific examples of such salts which may be used as reactants in this process are dimethyl methoxy magnesium malonate, diethyl ethoxy magnesium malonate, diphenyl and diben'zyl ethoxy magnesium malonate, ethyl t-butyl ethoxy magnesium malonate, and sodium, potassium, calcium and magnesium salts of 'dimethyl, diethyl, diphenyl, dibenzyl and similar mixed esters of malonic acid.

The reaction described may be used to prepare any desired diester of 4,6-(hydrocarbon substituted mercapto)- 2-carboxy-3-oxocaproic acid. In a specific illustration of this reaction u,'y-bis(methylmercapto)-butyryl chloride is reacted with ethyl t-butyl ethoxy magnesium malonate to obtain tertiary butyl 4,6-bis(methylmercapto)-2- carbethoxy-3-oxocaproate. Examples of similar compounds which are prepared in this manner from the appropriate reactants are methyl-4,6-bis(ethylmercapto)- 2 carbomethoxy 3 oxocaproate, ethyl 4,6 bis(ethylmercapto)-2-carbethoxy-3-oxocaproate, ethyl-4,6-bis(al lylmercapto)-2-carbethoxy-3-oxocaproate, methyl-4,6-bis (2-methyl-l-ethyl-2-butene mercapto)-2-carbethoxy-3-oxocaproate, propyl-4,6-bis(3-methyl-2-ethyl-2-pentene mercapto) 2 carbomethoxy 3 oxocaproate, phenyl 4,6- bis(phenylmercapt'o)-2-carbophenoxy-3-oxocaproate, ethyl 4,6 bis(benzylmercapto) 2 carbethoxy 3 oxocaproate, benzyl 4,6 bis(propargylmercapto 2 carbobenzyloxy 3 oxocaproate, methyl 4 ethylmercaptoy-6-propylmercapto-2-carbomethoxy-3-oxocaproate, and the like.

- According to the second step of this process the di- L roate, methyl-4-ethylmercapto-6-propylmercapto-3-ox0- caproate and the like.

As has been indicated above, this decarboxylation is also effected by thermal cracking. According to this procedure diesters of the 4,6-(hydrocarbon substituted mercapto)-2-carboxy-3-oxocaproic acids are heated at a suitable elevated temperature, preferably in the neighborhood ofabout ISO-300 C., to accomplish removal of the 2-carboxy group and form the corresponding monoesters. After the conversion has been accomplished the product may be recovered by conventional methods'and Acid catalyzed decarboxylation is preferably achieved 1 under solvent conditions in the presence of a suitable acid. Thus, this reaction is conveniently effected by contacting the diester of the 4,6-(hydrocarbon substituted mercapto)- 2-carboxy-3-oxocaproic acid in inert solvents such as ethers, benzene, hexane, chloroform and the like with catalytic amounts of acids such as alkyl and aryl sulfonic acids and particularly para-toluene sulfonic acid. In this regard, the use of substantially anhydrous conditions is sometimes preferable for optimum results. Elevated temperatures above 50 C. and up to the reflux temperature are used for the decarboxylation. Under such conpurified. By proper-selection of the reactants those mono-esters previously namedherein and other similar compounds can be produced by this procedure.

The esters of 4,6-(hydrocarbon substituted mercapto)- 3-oxocapr0ic acids are readily converted to the corresponding alkali metal and alkaline earth metal salts such as sodium, potassium, calcium and magnesium salts by hydrolysis of the ester with an alkali or alkaline earth metal base. The salts in turn are converted to the free acids by hydrolysis with mineral acids such as hydrochloric acid and sulfuric acid.

The third reaction of ,this process is directed to the production of various derivatives of 6,8-(hydrocarbon substituted mercapto)-4-carboxy-5-oxocaprylic acids such as nitriles, amides, diesters and dimetal salts thereof (VII). Such compounds may be conveniently prepared by reacting an ester or salt of a 4,6-(hydrocarbon substituted mercapto)-3-oxocaproic acid (V) prepared as above with a derivative of acrylic acid (VI) under suitable reaction conditions. This reaction may be readily effected by contacting the reactants in the presence of a suitable solvent, preferably an alcohol such as the lower alcohols like methanol and ethanol. Ordinarily a small amount of a quaternary ammonium hydroxide such as trimethyl benzyl ammonium hydroxide (Triton B) or an alkali metal alkoxide is added to the mixture to promote the reaction. To prevent undue side effects the temperature is ordinarily controlled below 60 C. After the reactants have been combined, preferably using approx- This converimately equimolar amounts of each reactant, the mixture is continually stirred for 12 to 60 hours during which time an elevated temperature below 60 C. is maintained. Upon completion of the reaction the desired product may be recovered from the reaction mixture by extraction with an immiscible solvent such as chloroform, evaporation of the solventzand distillation of the product under reduced pressure.

This reaction is conveniently effected with any suitable H derivative of acrylic acid such as acrylonitrile, acrylamide and esters and salts of acrylic acid such as the alkyl, aryl and aralkyl esters and alkali metal and alkaline earth metal salts thereof. More specifically, the methyl, ethyl, propyl, butyl,phenyl and benzyl esters and sodium, potassium, magnesium and calcium salts of acrylic acid are examples, of compounds which may be used in this reditions the reaction is completed in an hour or two and I action.

Specifically illustrative of this method is the reaction of ethyl 4,6 bis(methylmercapto) 3 oxocaproate with methyl acrylate to produce methyl-6,8-bis(methylmercapto) -4-carbethoxy-S-oxocaprylate. Other examples of closely related compounds that are incorporated within the scope of this invention which are produced according to this method from the appropriate reactants are ethyl 6,8 bis(ethylmercapto) 4 carbomethoxyoxocaprylate, ethyl 6,8 bis(ethylmercapto) 4- carbethoxy 5 oxocaprylate, methyl 6,8 bis (methylmercapto) 4 carbomethoxy 5 oxocaprylate, phenyl- 6, 8 bis(phenylmercapto) 4 carbethoxy -5 oxocaprylate, benzyl 6,8 bis(propylmercapto) 4 carbophenoxy 5 oxocaprylate, methyl 6,8 bis(allylmercapto)- 4 carbethoxy 5 oxocaprylate, ethyl 6,8 bis(2- methyl l ethyl 2 butene mercapto) 4 carbomethoxy S oxocaprylate, propyl 6,8 bis(3 methyl- 2 ethyl 2 pentenemercapto) 4 carbopropoxy 5- oxocaprylate, ethyl 6,8 bis(propynylmercapto) 4- carbethoxy 5 oxocaprylate, methyl 6 ethylmercapto 8 -propylmercapto 4 carbomethoxy 5 oxocaprylate and alkali metal and alkaline earth metal salts such as the sodium and potassium, calcium and magnesium salts and the nitrile and amide of these and similar compounds. The free acids of such compounds are readily obtained by hydrolysis of the nitrile, amide, esters and salts according to conventional methods.

The last reaction in this process comprises the combined hydrolysis and decarboxylation of a derivative of 6,8 (hydrocarbon substituted mercapto) 4 carboxy- 5-oxocaprylic acid (VII), such as described above, to the corresponding 6,8-(hydrocarbon substituted mercapto)- S-oxocaprylic acid (VIII). This reaction may be conveniently carried out by treating the reactant with a suitable mineral acid, preferably in the presence of a carboxylic acid. The reaction, however, is preferably run in a mixture of glacial acetic acid and hydrochloric acid since the product can be readily recovered by distillation of such acids. The reaction proceeds at room temperature although elevated temperatures are also used with satisfactory results. Under such conditions the reaction is completed in from about 6 to about 60 hours. The reaction product is readily recovered from the mixture according to conventional methods such as evaporation and extraction with a suitable solvent such as chloroform.

The described reaction is specifically illustrated by the conversion of methyl-6,8-bis(methylmercapto)-4-carbethbxy-S-oxocaprylate in the presence of a mixture of hydrochloric acid and glacial acetic acid to 6,8-bis(methylmercaptol-S-oxocaprylic acid. In a similar manner other compounds such as 6,8-bis(ethylmercapto)-5-oxocaprylic acid, 6,8-bis(propylmercapto)-5-oxocaprylic acid, 6,8-bis (butylmercapto)-5-oxocaprylic acid, 6,8-bis(phenylmercapto)-5.-oxocaprylic acid, 6,8-.bis(benzylmercapto)-5-oxocaprylic acid, 6,8-bis(allylmercapto)-5-oxocaprylic acid, 6,8 bis(2 methyl 1 ethyl '2 butene mercapto)- 5 oxocaprylic acid, 6,8 bis(3. methyl 2 ethyl 2- pentene mercapto) 5 oxocaprylic acid, 6,8 bis(propargylmercapto)-5-oxocaprylic. acid, 6-ethylmercapto-8- propylmercapto-S-oxocaprylic acid and the like are prepared by the proper selection of reactants.

Esters of the 6,8-(hydrocarbon substituted mercapto)- S-oxocaprylic acids are conveniently prepared by reacting such compounds with a suitable halogenat-ing agent to form the correspondingacyl halide and subsequently reacting the acyl halide with an alkanol, aryl alcohol or aralkanol to obtain the desired ester. The acyl halides of the 6,8-(hydrocarbon substituted Inercapto)5-oxocaprylates may be conveniently prepared by known methods such. as by reacting the acid with halogenating agents such as thionyl chloride, phthalyl chloride, phosphorous trichlor-ide, thionyl bromide and phosphorus tribromide in the presence of conventional solvents. or an excess of the halogenating agent if it is a liquid. This halogenation may be achieved at 0-10 C. or at a more'elevated'temperature if desired. After the acyl halide has been prepared it is recovered from the reaction mixture and esterified by reaction with an excess of the appropriate alcohol under acidic conditions. The esters formed in this manner are recovered from the reaction mixture by ordinary procedures.

Examples of some 6,8-(hydrocarbon substituted mercapto)-5-oxocaprylic acid esters which may be prepared from the corresponding acid and intermediate acyl halides such as the bromide and chloride that might be mentioned are methyl-6,8-bis(methylmercapto)-5-oxocaprylate, methyl-6,8-bis(ethylmercapto)-3-oxocaprylate, ethyl- 6,8 bis(ethylmercapto) S-oxocaprylate, benzyl-6,8-bis (propylmercapto) S-oxocaprylate, propyl-6,8-bis(butylmercapto)-5-oxocaprylate, phenyl-6,8-bis(phenylmercapto) 5-oxocaprylate, methyl-6,8-bis(benzylmercapto)-5- oxocaprylate, methyl-6,8-bis allylmercapto -5-oxocapryl ate, ethyl 6,8 bis(propargylmercapto)-5-oxocaprylate, methyl 6-ethylmercapto-8-propylmercapto-S-oXocaprylate and the like.

Lower alkyl esters are also conveniently produced by reacting a 6,8-(hydrocarbon substituted mercapto)-5- oxocaprylic acid with a diazoalkene, such as diazomethane or diazoethane, in a suitable dry inert solvent such as ether, chloroform or benzene. Esters may also be produced by reacting the acid with a suitable alcohol under acidic conditions according to the classical method of producing esters. The resulting esters, like the acids are water insoluble oils which may be purified by distillation under reduced pressure.

In addition to esters, salts of the 6,8-(hydrocarbon substituted mercapto)-5-oxocaprylic acids may also be produced. For example, the alkali metal and alkaline earth metal Salts may be prepared by contacting a suitable organic solvent solution of a 6,8-(hydrocarbon substituted mercapto)-5-oxocaprylic acid with an aqueous solution of an alkali metal or alkaline earth metal hydroxide, bicarbonate or carbonate. The salt is readily recovered from the aqueous phase by evaporation to dryness. In this manner, salts such as the sodium, potassium, calcium and magnesium salts of 6,8-(hydrocarbon substituted mercapto)-5-oxocaprylic acids, such as those specifically named hereinbefore, may be prepared.

According to a further embodiment of this invention diesters of 6,8-(hydrocarbon substituted mercapto)-4- carboxy-S-oxocaprylic acids may also be produced by reacting an ester of a 4,6-(hydrocarbon substituted mercapto)-3-oxocaproic acid with an ester of a B-halo propionic acid. This reaction may be illustrated as follows:

| SR SR ll CHsCHzCHC CHCHzCHaCO O R SR SR C O O R wherein R and R represent the same or dilferent hydrocarbon substituents such as alkyl, alkenyl, alkynyl, aryl and aralkyl groups, and R and R represent the same or different alkyl, aryl and aralkyl groups, and X is halogen.

This reaction is preferably elfected by contacting the reactants in the presence of a suitable inert organic solvent which is anhydrous or substantially free of water; Among the solvents which may be used for this purpose are ethereal solvents such as dioxane, tetrahydrofuran and ether alcohols such as methanol and ethanol, benzene, toluene and hexane. Approximately, equimolar quantities of the reactants are preferably employed in the reaction together with about equimolar quantity of an enolizing agent such as sodamide, sodium triphenylmethyl and sodium and potassium alkoxides such as methoxide or ethoxide. The reaction is conventiently accomplished at elevated temperatures such asthe reflux temperatures of the mixture. Under such conditions the reaction is ordinarily completed in from about one to five hours, usually two hours being entirely adequate. Following completion of the reaction the mixture is preferably acidified with a mineral acid such as sulfuric acid or hydrochloric acid to decompose any excess base present and to aid the recovery of the desired product. The product may then be recovered according to theordinary techniques.

This reaction is conveniently accomplished employing esters of fi-halo propionic acid in which the ester substituent is an alkyl, aryl or aralkyl group such as methyl, ethyl, propyl, phenyl and benzyl radicals and in which the B-halo substituent is chlorine or bromine.

The application of this alternative procedure may be conveniently used to prepare those diesters of 6,8-(hydrocarbon substituted mercapto)-4-carboxy-5-oxocaprylic acids previously disclosed herein and other similar novel compounds.

The 6,8-(hydrocarbon substituted mercapto)-5-oxocaprylic acids produced according to the present invention may be converted to 5-[3-(1,2-dithiacyclopentyl)] pentanoic acid, also called ot-lipoic acid, by the utilization of aprocess which is the joint invention of Holly and Wagner and is disclosed in United States patent ap- 'plication Serial No. 396,333, filed December 4, 1953, now U.S. Patent No. 2,853,497. This process comprises essentially reacting a 6,8-(hydrocarbon substituted mercapto)-5-oxocaprylic acid with an alkali metal borohydride to produce the corresponding 6,8-(hydrocarbon substituted mercapto)-5-hydroxy caprylic acid which immediately forms the corresponding S-membered lactone, reducing said lactone with phosphorous-iodine to produce a 6,8-(hydrocarbon substituted mercapto) caprylic acid, and reacting said compound with dealkylating agents such as 50% sulfuric acid or thiouronium bromide and oxidizing the resulting material with I KI to obtain 5-[3- (1,2-dithiacyclopentyl)] pentanoic acid.

In the practice of the present invention, the an -(hydrocarbon substituted mercapto) butyryl halides which are used as starting materials may be conveniently prepared according to novel methods disclosed in the United States patent application of Arthur F. Wagner, Serial No. 369,535, filed July 21, 1953, now U.S. Patent No. 2,842,- 587.

The following examples are given for purposes of illustration and not by way of limitation: 1

EXAMPLE 1 Tertiary butyl 4,6-bis(methylmercapto)-2-carbeth0xy-3- oxocaproate A suspension of 0.10 mole of magnesium ethylate in 100 ml. of anhydrous ether is stirred While 0.10 mole of ethyl t-butyl-malonate is added dropwise over a period of five minutes. The mixture is then refluxed for 15 minutes. To the resulting solution of ethyl t-butyl ethoxy magnesium malonate is added a solution of 0.12 mole of a,'y-bis(methylmercapto) butyryl chloride 'in 75 ml. of ether over a ten minute period. The mixture is then refluxed for 15 minutes and cooled. The mixture containing t-butyl 4,6-bis(met-l1ylmercapto)-2-carbethoxy-3- oxocaproate is diluted with 60 ml. of water and acidified with 20 m1. of 2 N sulfuric acid. 'The ether .phase is separated and the aqueous phase is extracted with ether. The combined ether extracts are washed with water, dried over anhydrous magnesium sulfate, and the ether removed. Final traces of water are removed by azeotropic distillation with benzene.

The a,'y-bis(methylmercapto) butyryl chloride used in this example maybe conveniently prepared according to the following representative procedures using oxalyl chloride and thionyl chloride as the halogenating agents:

(A) About 10.7 grams of sodium a,'y-bis(methylmercapto) butyrate is suspended in ml. of dry benzene and cooled in an ice bath. To this mixture is added 10.2 grams of oxalyl chloride over a ten minute period and the mixture is stirred for one hour at ice bath temperatures. The mixture is then evaporated under reduced pressure at a temperature below 15 C. The residue is treated with two -ml. portions of ether, the ether extracts filtered and evaporated under reduced pressure to give a,'y-b-is(methylmercapto) butyryl chloride as an oil. (B) About 13.29 grams of oc,'y-bis(methylmercapto) butyric acid is cooled in an ice bath and 9.7 grams of thionyl chloride added thereto. After standing at about 0 C. for one hour the mixture is evaporated at room temperature under reduced pressure to remove thionyl chloride and volatile side products. The a,'y-bis(methylmercapto) butyryl chloride is obtained as an oil.

EXAMPLE 2 Tertiary butyl-4,6-bis(t-butylmercapto)-2 carbethoxy-.i-oxocaproate In ml. of anhydrous ether 18.8 grams of ethyl t-butyl malonate is reacted with 11.4 grams of magnesium ethylate to produce ethyl t-butyl ethoxy magnesium malonate. To this solution is then added 33.8 grams of a,' -bis(t-butylmercapto) butyryl chloride in 100 ml. of ether and the mixture refluxed for one hour. Tertiary butyl- 4,6 bis(t butylmercapto) 2 carbethoxy 3-. oxocaproate is obtained and is isolated as in Example 1.

' EXAMPLE 3 Tertiary butyl-4,6-bis(allylmercapto)-2-carbethoxy- 3-oxocaproate EXAMPLE 4 Tertiary butyl4,6-bis(benzylmercapto)-2- carbomethoxy-3-oxacaproate Following the procedure of Example 1, 26.1 grams of methyl t-butyl malonate is reacted with 17.1 grams of magnesium ethylate in 100 ml. of ether to form methyl t-butyl ethoxy magnesimum malonate. The mixture in turn is combined with 59.6 grams of a,-y-bis(benzylmercapto) butyryl chloride in 100 ml. of ether and reacted according to this procedure to produce t-butyl4,6-bis- (benzylrnercapto) -2-carb omethoxy-3-oxo caproate.

EXAMPLE 5 E thyl-4,6-bis(methylmercizpto)-3-0x0caproate After substantially all the water is removed from the t butyl 4,6 bis(methylmercapto) 2 carbethoxy 3- oxocaproate prepared as in Example 1 the benzene solution is allowed to cool and one gram of anhydrous p-toluene sulfonic acid is added. The acidic solution is re fluxed for 1 /2 hours after the initial vigorous evolution of isobutylene and carbon dioxide has ceased. The benzene solution is then washed with saturated aqueous sodium bicarbonate and with water. The benzene solution is dried over magnesium sulfate and the benzene removed by evaporation under reduced pressure. The oily residue is distilled to obtain ethyl-u,'y-bis(methylrnercapto)-3- oxocaproate; boiling point -114 C./.05 mm.;' n =l.5088; 7t max.=2940 A. (E% =480) and 2460 g0E%=1220); molecular weight found 255:3 (calc.

9 EXAMPLE 6 Ethyl-4,6-bis(t-bulylmercapto)-3-0x0capr0ate To 30 grams of t-butyl-4,6-bis(t-butylmercapto)-2- carbethoxy-3-oxocaproate in 100 m1. of anhydrous benzene is added 1.5 grams of p-toluene sulfonic acid and the mixture refluxed for two hours. Ethyl-4,6-bis(tbutylmercapto)-3-oxocaproate is formed and recovered as inExample 5.

EXAMPLE 7 Ethyl-4,6-bis(allylmercapto) -3-0x0caproate To 30 grams of t-butyl-4,6-bis(allylmercapto)-2-carbethgxyfieoxocaproate in 200 ml. of benzene substantially free of water is added 3 grams of p-toluenesulfonic acid. The benzene solution is refluxed for two hours, washed with aqueous sodium bicarbonate and dried over magnesium sulfate. Ethyl-4,6 bis-allylmercapto)-3roxocaproate is recovered by evaporating the benzene.

EXAMPLE 8 Tertiary butyl-4,6-bis( benzylmercapto -3-0xocaproate About, 40 grams of t-butyl-4,6-bis (benzylmercapto) -2- carbornethoxy-3-oxocaproate is dissolved in 150 ml. .of anhydrous benzene and two grams of p-toluenesulfonic acid is added. Reaction is etfected as in Example 5 to produce t-butyl-4,6-bis(benzylmercapto)-3-oxocaproate which is recovered as therein disclosed.

EXAMPLE 9 Methyl-6,8-bis(methylmercapto) -4-carbethoxy- 5 -x0caprylate A solution of 20 grams of ethyl-4,6-bis-(methylrnercapto)-3-oxocaproate and five grams of 30% methanolic trimethyl benzyl ammonium hydroxide (Triton B) is stirred while seven grams of methyl acrylate is added over a 15-minute period. During this addition the temperature rises to about 35 C. and soon increases to 50 C. The reaction is held at about 50 C. for 48 hours after which it is cooled and extracted with chloroform. The chloroform solution is washed with 50 ml. of 1 N hydrochloric acid, water and finally dried over anhydrous magnesium sulfate. The ether is removed and the residual oil is distilled to obtain methyl-6,8-bis(methyhnercapto)-4-carbethoxy-S-oxocaprylate.

EXAMPLE 10 M ethyl-6,8-bis( t-butylmercapto -4-carbelh0xy- -0x0caprylate A methanol solution of 25.1 grams of ethyl-4,6-bis(tbutylmercapto)-3-oxocaproate is prepared and to it is added 6.4 grams of methyl acrylate and 4 grams of 30% methanolic trimethyl benzyl ammonium hydroxide. The mixture is maintained at about 50 C. for 20 hours and the methyl 6,8 bis(t butylmercapto) 4 carbethoxy- 5-oxocaprylate recovered by the procedure of Example 9.

EXAMPLE 11 Methyl-6,8-bis(allylmercapto) -4-carbethoxy-5- oxocaprylate To a solution of 36 grams of ethyl-4,'6-bis(allylmercapto)-3-oxocaproate in methanol is added 10.5 grams of methyl acrylate and 7 grams of 30% methanolic trimethyl benzyl hydroxide. The mixture is kept at about 50 C. or one day and then extracted with ether. The product is worked up in the usual manner to obtain methyl 6,8 bis(allylmercapto) 4 carbethoxy 5- oxocaprylate.

EXAMPLE 12 Methyl-6,8-bis(benzylmercapto)-4-carb0-tbutoxy-S-oxocaprylate By reacting 19.4 grams of 5-butyl-4,6-bis(benzyhnercapto)-3-oxocaproate with 4.3 grams of methyl acrylate in ethanol in the presence of 3 grams of 30% methanolic trimethyl benzyl ammonium hydroxide according to Example 9 there is produced methyl-6,8-bis(benzylmercapto)-4-carbo-t-butoxy-S-oxocaprylate which may be isolated according to conventional procedures.

EXAMPLE 13 6,8-bis(methylmercapt0)-5-0x0caprylic acid A solution of 6.8 grams of methyl-6,8-bis(methylmercapto)-4-carbethoxy-S-oxocaprylate in 100 ml. of glacial acetic acid and 24 ml. of concentrated hydrochloric acid is prepared and allowed to stand at room tempera ture for 48 hours. The mixture is concentrated under reduced pressure, the residue extracted with aqueous, sodium bicarbonate, the extract acidified and extracted with chloroform. The chloroform is removed and the residue is dissolved in 50 ml. of cold 1 N sodium hydroxide and allowed to stand at about 6 C. overnight. The mixture is acidified to pH 3 with concentrated hydrochloric acid and extracted with chloroform. The chloroform is removed and the product heated at 85 C. for 1 /2 hours to eliminate carbon dioxide. The resulting 6,8-bis(methylmercapto)-5-oxocaprylic acid has ultra violet absorption maxima at 2390 A. (E% =27.7) and 2970 A. (E%=15.5).

EXAMPLE 14 6,8-bis(t-butylmercapto)-5-0x0caprylic acid Methyl 6,8 bis (t butylmercapto) 4 carbethoxy-S-oxocaprylate is reacted according to the method of Example 13 to form 6,8-bis(t-butylmercapto)-5-oxocaprlyic acid.

EXAMPLE 15 6,8-bis(allylmercapto -5-oxocaprylic acid Ten grams of methyl-6,8-bis(allylmercapto)-4-carbethoxy-S-oxocaprylate is added to an equeous hydrochloric acid solution and the mixture is allowed to stand for one day. The aqueous mixture is extracted with choroform and by evaporating the chloroform solution 6,8- bis(allylmercapto)-5-oxocaprylic acid is isolated.

EXAMPLE 16 6,8-bis(benzylmercapto)-5-0x0caprylic acid EXAMPLE 17 Methyl-6,8-bis(methylmercapzo)-carbeth0xy-5- oxocaprylate using methyl-fi-chloropropionate To a solution of 4.5 grams of sodium methoxide in ml. of methanol is added 20 grams of ethyl-4,6- bis(methylmercapto)-3-oxocaproate. The resulting solution is refluxed and to it is added 9.8 grams of methylfi-chloropropionate over a ten-minute period. Refluxing is continued for two hours With stirring. The mixture is cooled and acidified to pH 3-4 with a cold solution of hydrochloric acid. The acidified mixture is diluted with ice water and extracted with chloroform. The chloroform extracts are washed with water and dried over anhydrous magnesium sulfate. The chloroform is evaporated and the residual oil distilled to give substantially pure methyl 6,8-bis(methylmercapto)-4-carbethoxy- S-oxo-caprylate; boiling point 160 C./50 mm., 11 =1.5028.

Any departure from the above description which conforms to the present invention is intended to be included within the scope of the claims.

11 What is claimed is: l. A compound having the formula:

omcmofi cnomomcoolv SR SR 00R4 wherein R and R are selected from the class consisting of lower alkyl, lower alkenyl, phenyl and benzyl groups, R represents a member of the group consisting of hydrogen, lower alkyl, aryl and aralkyl groups and alkali and alkaline earth metals.

2. A lower alkyl ester of 6,8-bis(lower alkylmercapto)- 4-carbo-1ower alkoxy-S-oxo-caprylic acid.

3. Methyl 6,8 bis(methylmercapto)-4-carbethoxy- 5-oxocaprylate.

4. Methyl 6,8 bis(allylmercapto) 4 carbethoxy- 5-oxocaprylate.

5. The process which comprises hea 'ng a compound of the formula cmcmcm ioflzooom SR SR with a compound of the formula CH =CHCOOR in a liquid solvent medium at a temperature of less than about 0., thereby producing a compoundhaving the formula wherein R and R are selected from the class consisting of lower alkyl, lower alkenyl, phenyl and benzyl groups, and R is a lower alkyl group.

6. A process which comprises heating ethyl-4,6-bis- (methylmercapto)-3-oxocaproate with methyl acrylate-in a liquid solvent medium at a temperature of less than about 60 C. thereby producing methyl-6,8-bis(methylmercapto) -4-carb ethoxy-S -oxocaprylate.

7. A process which comprises heating ethyl-4,6-bis allylmercapto)-3-oxocaproate with methyl acrylate in a liquid solvent medium at a temperature of less than about 60 C. thereby producing methyl-6,8-bis(allyl.mercapto)-4-carbethoxy-S-oxocaprylate.

References Cited inthe file of this patent Connor et al.: J. Org. Chem., vol. 3, p. 570 (1939). v Surrey: Name Reactions in Organic Chemistry, p. (1954). 

1. A COMPOUND HAVING THE FORMULA: 